You can paint, POR-15 or do whatever you like over powdercoat. Painting it may require your typical pre-paint prep beforehand though. But from my past experiences and testing, powdercoating is way superior in rust preventative compared to POR-15, primer or other rust preventative stuff I've used. So it'll be like rubber-coating a plastic sheet to make sure it gets waterproof (plastic is waterproof to begin with), not really necessary. Just my 2 cents...
I used to work in the powder coatings industry so I know "little bit" about it.
I did research and development / manufacturing of powder coatings for about 5 years.
Topcoat adhesion to powder coating really depends on the type of powder that was initially used and the amount of gloss of powder coating has. A high-gloss polyester or a urethane powder coating is tough to overcoat with any kind of paint and get good adhesion. A gloss of usually 40 or lower is required for any kind of topcoat to stick to powder. They can be done however by lightly standing to powder coating with 220 grit, use a good primer and then topcoat. It is usually best just leaving powder well enough alone though as It's the best metal coating found.
I think POR 15 is basically designed to go over metals. A chemically bonds to the metal which will not work on a plastic powder coating. To do it properly (por15) you need to use their special etching primer/ wash, this won't have any effect on a powder coating either. I used this stuff on the thing the arms and spindles for my baja and it is tough as nails. A little bit goes a loooong way.
